How to assembly a matrix using vectors?

Its not fun to debug a picture. You don't even show what the error message says. We don't know anythng about the variables you are using - whether you use units or not, etc.

Please atach your worksheet.

BTW, your questions obviously was asked two days ago but is just showing up now. This forum seems to be screwed up at the time.

Have you got the subscript f in the "return" Q5 implemented as an index rather than a name subscript? Its difficult to tell from the image, but it looks like the Q is red but the f is black ...

Stuart